Adding a quick link to the source code / schema of example at docs

Hi team,

I am trying to learn `GraphQL` by reading [the docs](https://graphql.org/).

While I personally found the [interactive examples](https://graphql.org/learn/queries/#fields) quite useful, I was also quite curious about the actual schema / data behind the interactive examples.

As discussed in #728, I believe [the schema and data can be found here at `graphql.github.io/site/_core/swapiSchema.js`](https://github.com/graphql/graphql.github.io/blob/739a1c913fa4a88ac7c64c93ce68fc64ec0cbd96/site/_core/swapiSchema.js#L21)

However, I wonder if we could make the documentation more user-friendly by adding a quick link at the top of the documentation page, say, something like `You can find the schema and data behind these interactive examples here`.

Anyway, thanks again for your tremendous efforts in maintaining the library and docs!

Start at the interactive examples in the Learn Queries documentation, especially the Fields section, and inspect the linked site/_core/swapiSchema.js schema and data. Done means the documentation page includes a quick link explaining where the schema and data behind the examples can be found.

Scritto dal modello di indicizzazione a partire dal testo della issue.